1. Field of the Invention
This invention relates to a control system that controls a controlled object to which is applied periodic disturbance the amplitude of which periodically changes.
2. Description of the Related Art
Conventionally, a control system that controls a variable cam phase mechanism of an internal combustion engine has been disclosed in Patent Literature 1 (Japanese Laid-Open Patent Publication (Kokai) No. 2001-132482). This variable cam phase mechanism changes the phase of an intake camshaft, i.e. an intake cam, with respect to a crankshaft (hereinafter referred to as “the cam phase”) as desired to thereby change the valve timing of intake valves, and is comprised of a hydraulically-driven variable cam phase mechanism, and a solenoid control valve for supplying hydraulic pressure from an oil pump to the variable cam phase mechanism. Further, the control system includes a crank angle sensor and a cam angle sensor which output signals indicative of the angle position of the crankshaft and that of the intake cam, respectively, and a controller to which are inputted the detection signals from the sensors.
The controller calculates an actual cam phase based on the detection signals from the crank angle sensor and the cam angle sensor, and a target cam phase depending on operating conditions of the engine, and as described hereinafter, controls the variable cam phase mechanism with a sliding mode control algorithm such that the cam phase is caused to converge to the target cam phase. In other words, the controller regards a system to which is inputted a control signal for the solenoid control valve as a control input and from which is outputted the cam phase, as a controlled object, and models the controlled object into a continuous-time system model. More specifically, the characteristic equation of the controlled object is set as a differential equation in which the first-order and second-order time derivative values of the cam phase are represented as state variables. Further, a switching function of the sliding mode control algorithm is set as a linear function in which the difference between the target cam phase and the cam phase and a time derivative value of the difference (i.e. the rate of change in the difference) are represented as state variables. Then, the control input is calculated such that the difference and the rate of change in the difference set as above as the state variables of the switching function are on a switching line. In other words, the control input is calculated such that the difference and the rate of change in the difference slide on the switching line to converge to a value of 0, whereby the cam phase is caused to converge to the target cam phase.
Further, a control system using the sliding mode control algorithm has been proposed in Patent Literature 2 (Japanese Laid-Open Patent Publication (Kokai) No. 2003-5804) by the present assignee. This control system controls a throttle valve-actuating mechanism for the engine, and includes an adaptive sliding mode controller, an onboard identifier, a state predictor, and so forth. Further, the throttle valve-actuating mechanism actuates a throttle valve to thereby change the degree of opening thereof, and includes a motor.
In the control system, a control input for control of the throttle valve-actuating mechanism is calculated as follows: A system to which is inputted the duty ratio of a control signal supplied to the motor as a control input and from which is outputted the difference between the degree of opening of the throttle valve and a target degree of opening thereof is regarded as a controlled object, and the controlled object is modeled into a discrete-time system model defining the relationships between the duty ratio, the difference between the degree of opening of the throttle valve and the target degree of opening thereof, and a compensation value. The compensation value is for compensating for modeling errors in modeling the controlled object, and disturbance.
Then, model parameters of the controlled object model and the compensation value are calculated for identification by the onboard identifier, and the control input is calculated by the adaptive sliding mode controller, using the above identified values, with the sliding mode control algorithm. In the control system, since the control input is calculated as above, it is possible to properly compensate for the modeling errors and the disturbance, thereby making it possible to ensure high accuracy of control.
The control system proposed in Patent Literature 1 suffers from the following problems: (f1) The influence of disturbance on the controlled object is not taken into account, and hence when the controlled object is a variable cam phase mechanism which is liable to be subjected to a steady-state disturbance, the stability and the accuracy of control is degraded by the steady-state disturbance. (f2) Further, the variable cam phase mechanism is provided for changing the phase of the intake cam with respect to the crankshaft, as desired, and hence when the intake cam actuates the intake valve to open and close the same, the intake cam is subjected to a periodic disturbance the amplitude of which periodically changes, due to the urging force and the reaction force of a valve spring of the intake valve (see FIG. 12, referred to hereinafter). When such a periodic disturbance is applied to the variable cam phase mechanism, the total valve open time period of the intake valve is shortened by the influence of the periodic disturbance (see FIGS. 14 and 15, referred to hereinafter), and the amount of intake air decreases when the intake valve is opened. This reduces torque generated by the engine to make unstable the combustion state of the engine.
(f3) Further, since the continuous-time system model is used as a controlled object model, it is difficult to directly identify model parameters of the controlled object model from experimental data of the controlled object. For this reason, it is necessary, more specifically, to approximately transform the continuous-time system model to a discrete-time system model to identify the model parameters based on the discrete-time system model. The use of such approximate transform degrades the accuracy of identification of the model parameters. Furthermore, it is required to approximately transform the discrete-time system model to the continuous-time system model again, which causes an increase in modeling errors occurring in modeling the controlled object. Consequently, to ensure a large margin of the stability of the control, it is necessary to reduce the controller gain, resulting in further degraded controllability. In short, the control system proposed in Patent Literature 1 cannot ensure the robustness and the response-specifying characteristics peculiar to the sliding mode control.
To solve the above described problems in Patent Literature 1, it is contemplated to apply the control method in Patent Literature 2 to the control system proposed in Patent Literature 1. In this case, although the above-described problems (f1) to (f3) in Patent Literature 1 can be solved, since the control method in Patent Literature 2 calculates the compensation value by the onboard identifier, it is impossible to solve the problem (f2) until the number of times of the calculation reaches a predetermined value. In short, it takes some time to compensate for and suppress the influence of the periodic disturbance, which can degrade the stability and the accuracy of the control during the time.